1. Field of the Invention
The present invention relates to a vulcanizing mold and vulcanizing method for pneumatic tires.
2. Description of the Related Art
Conventional vulcanizing molds for pneumatic tires are typically classified into two types, i.e., a full-type mold and a split-type mold. A full-type mold has inner surface portions for forming the tread portion and side portions of a tire, and is divided into upper and lower mold halves at a parting plane which corresponds to the tire equatorial plane. These mold halves are displaceable in the vertical direction toward and away from each other for closing and opening the mold, respectively.
The full-type mold is advantageous in that it is simple in structure, small in size and inexpensive. However, upon removal of a product tire out of the mold after completion of vulcanization, the mold is opened only by a relative vertical displacement of the upper and lower mold halves, and the full-type mold may thereby give rise to formation of defective products depending upon the tread pattern. More particularly, during the relative vertical displacement of the mold halves, the projections or ridges provided on the mold inner surface for forming the tread portion of the tire may exert excessive shearing forces to the tread land regions, thereby causing such problem that defects tend to be formed in the tread land regions such as nicks and/or cracks.
On the other hand, a split-type mold is comprised of upper and lower sidewall mold members which are vertically displaceable relative to each other, and a tread mold member which is divided into a plurality of arcuate segments arranged successively in the circumferential direction. The segments of the tread mold member are radially displaceable to increase or decrease the diameter.
In such a split-type mold, as shown in FIG. 1, in order to remove the product tire T out of the mold after completion of vulcanization, it is necessary to displace the respective segments C radially outward so as to increase the diameter. On this occasion, the abutment surface b of the segment C has to be displaced from the corresponding abutment surfaces of the upper and lower sidewall mold members SU and SL at least by an amount d, in order to positively disengage the tread land region of the tire T from the projections or ridges on the inner surface of each segment and avoid interference therebetween, for preventing formations of defects in the tread land region such as nicks and/or cracks. It is further necessary to subsequently displace the segments in predetermined directions, typically vertically upwards, to respective positions where they do not hinder the removal of the tire T from the mold. As such, it is inevitable that the split-type mold as a whole is large in its outer diameter. This is particularly the case where tires having a large aspect ratio are to be produced, such as off-road tires and motorcycle tire, because the size of the mold, hence the size of vulcanizing machine, must be large with reference to the tire size, thereby resulting in increased cost for the entire facility.